Thrombopoietin (TPO) is a hepatocyte-derived growth factor known to promote platelet number, to have growth-promoting potential for megakaryocytes (MKs), and to promote erythrocyte, monocyte, mast cell, and granulocyte proliferation in the presence of specific growth factors. To explore the ability of TPO alone to support mast cell proliferation, we cultured CD34+ cells in rhTPO under conditions developed to culture MKs and examined these cultures for the presence of HuMCs. Similarly, we added rhTPO to CD34+ cells cultured in stem cell factor, known to promote HuMC development. Using serum-free media (SFM) supplemented with rhTPO, human CD34+ cells at 2-3 weeks differentiated into MKs (85-90%) and (5-10%) poorly differentiated. HuMCs (5-10%. SCF-independent HuMCs expressed TPO (c-Mpl) receptors, tryptase and chymase; and were capable of surviving in SFM with rhTPO alone, or proliferating when recultured in rhSCF. Small numbers of HuMC were noted when rhTPO was combined with 10 ng/ml rhSCF. However, in HuMC cultures with 100 ng/ml rhSCF, the addition of 50 ng/ml rhTPO significantly reduced HuMC numbers, with the appearance of MKs. We also examined TPO levels in patients with mastocytosis. These levels did not correlate with disease severity or platelet counts. Thus, TPO acting through its receptor c-Mpl, can support HuMC. The effect of TPO on SCF-dependent HuMC varies depending on the concentration of cytokines, with higher doses of rhTPO favoring MK proliferation. We found no evidence for a role for TPO in mastocytosis. Several human mast cell lines have been established which are dependent upon stem cell factor, double in number in 3 to 6 weeks, have the appearance of mature cells, and degranulate following aggregation of Fc gamma RI on their surface. These mast cells may also be sensitized with patient sera and activated by addition of allergen to which the donor of the serum was sensitive.